This invention relates generally to inductive devices, and more particularly to coils of fine wire and of very small dimensions suitable for use in hearing aids and other devices employing miniature circuitry.
In certain applications, such devices are referred to as telecoils and are installed in a hearing aid to sense the magnetic field of a telephone or other assistive listening system for the hearing impaired. The detection of such fields controls the operation of the electronic circuit of the hearing aid. In current practice, a telecoil typically consists of several thousand turns of fine insulated coil wire, typically on the order of one thousandth inch in diameter, wound on a ferrous or other core of magnetic material. In manufacture, the winding wire may be wound directly on a bobbin of magnetic material which forms a part of the telecoil, or the wire may be wound on a mandrel which is removed after winding, leaving a hollow core into which a ferrous rod is later inserted. The ultrafine coil wire is very fragile and is typically unsuited for connection to external circuit components for that reason. Therefore, it is typically necessary to provide heavier connection or lead wires that extend to such external circuit elements, the lead wires having, for example, five or six times greater diameter and being soldered or otherwise electrically connected to the ends of the fine winding wire. In these miniature devices terminal pads may be provided at one or both ends of the coil or cemented to the exterior body of the coil, and the fine wire may be wrapped around the lead wires which are in turn attached by adhesive or otherwise to the outside of the coil after winding.
One of the objects of the invention is to provide improved coils of minimized diameter and overall coil length.
Another object is to provide an improved structure whereby the lead wires are pre-mounted on the bobbin (or mandrel) prior to winding, thus providing to the winding equipment integral posts for coil wire terminations.
Another object is to provide an improved structure in which neither solder connections nor bare lead wires come into contact with the ultrafine coil wire of the winding.
Another object is to provide an improved construction that eliminates mechanical stress on the solder connections and increases the pull strength of the lead wires when connecting them to external circuit elements.
Another object is to provide a construction in which the lead or connection wires will only be subjected to bending in an area remote from the soldered area during connection of the coil to external circuit elements, as the soldered area typically becomes embrittled and weakened during soldering.
Another object is to provide the foregoing advantages to the coil using conventional winding methods but at reduced costs for parts, tooling and assembly.
Other objects of the invention will be understood from the following detailed description with reference to the appended drawings.
With the foregoing objects in view, this invention features lead wires that extend inwardly of the coil winding from end to end thereof, forming start and finish connection posts at one end of the coil around which the respective ends of the winding wire are wound. Advantageously, the lead wires are preformed and the coil winding is wound over the lead wires, the ends of the winding being extended out to the posts for winding on and electrical connection to the posts.
Another feature is that the foregoing construction can be achieved either by winding the fine coil wire on a bobbin of magnetic material that forms a part of the completed coil, or the fine wire may be wound on a removable mandrel which, after winding, is replaced by a ferrous or other magnetic core or rod.
Another feature is that the improved coil may be formed on any of several presently available winding machines in which the bobbin or mandrel is either rotating or non-rotating.
Other features of the invention and the achievement of other objects hereinabove referred to will be evident from the following description.